1. Field of the Invention
The present invention relates generally to synchronous machines, and, more specifically, the present invention is directed to permanent magnet synchronous machines with improved torque and/or power density characteristics.
2. Description of the Background
Recent advancements and applications of at least three technologies—microprocessors, power semiconductors and rare earth permanent magnets—has significantly contributed to the development of permanent magnet synchronous motors. Permanent magnet motors are typically characterized by the distinct advantages of simple construction, high efficiency, high power factor, and high power density. These types of motors are therefore well suited to ship and other vehicle propulsion systems.
As the size and weight of vehicles has rapidly increased, the development of advanced propulsion systems has inherently involved the design and construction of advanced large power, low speed propulsion motors. Space and weight constraints play a major role in the development and selection criteria. That is, it is generally advantageous to maximize the torque density or torque per unit weight of the motor.
Over the past several years, high performance, high efficiency, and lightweight electrical motors using advanced magnetic materials have been developed. Permanent magnet (PM) synchronous machines take advantage of both the advances in magnetic materials and electronic design to provide improvements in torque and power density. These permanent magnet motors have been designed and developed for many different electromagnetic configurations and approaches.
There are several types of rotor structures or topologies for PM synchronous motors. In general, the structures of the rotor in these machines can be divided into radial and transversal or circumferential forms depending on the orientation of the magnetization direction of the magnet. FIG. 1 shows a block diagram of PM synchronous machines comprising both a radial (FIG. 1(A)) and a circumferential (FIG. 1(B)) rotor structure. The basics components of each machine are generally the same.
As seen in FIG. 1(A), a radial-oriented rotor PM synchronous machine is characterized by a magnetic rotor core 10 with a plurality of radial-oriented permanent magnets 15 spaced around the outside of the rotor core 10. The iron core stator 20 with circumferential stator teeth 25 is oriented around the outside of the rotor, separated by an air gap. The flux path through the air gap of the radial oriented synchronous machine is shown as arrows 30. In the radial magnet arrangement of FIG. 1(A), the permanent magnets 15 operate in series.
For applications requiring high power density and performance, that is, a smaller motor volume for a given torque or power rating, with a comparatively high efficiency and power factor, the circumferential structure shown in FIG. 1(B) is potentially more suitable than the radial structure of FIG. 1(A). The circumferential-oriented magnet design is characterized by a non-magnetic rotor core 50 with alternating circumferential-oriented permanent magnets 55 and iron rotor pole pieces 60 around the outside of the rotor core 50. The stator may be the same as in the radial-oriented magnet design. In this circumferential arrangement, two permanent magnets act together to supply the air gap flux. A higher air gap flux density can thus be obtained for deep magnet configurations. Since the magnetic shear stress and rating of the motor are proportional to the air gap flux density, a smaller and more power dense machine typically results.
FIG. 2 shows a detailed view of a conventional motor pole geometry arrangement using circumferential-oriented permanent magnets. The stator is comprised of a stator core backiron 70 held in place by the stator housing 75 and including a plurality of stator windings 80 aligned vertically along the radial axis of the motor. The rotor is comprised of alternating circumferential-oriented permanent magnets 85 and iron rotor pole pieces 90 around the outside of the rotor housing 92. As seen in FIG. 2, rectangular-shaped permanent magnets 85 are used in conventional circumferential-oriented magnet arrangements. Therefore, in order to hold the permanent magnets 85 in place against the centrifugal forces of the spinning motor, the rotor pole pieces 90 are bolted to the rotor housing 92, and an interpole wedge 95 is secured into slots near the top of adjacent rotor pole pieces 90. This interpole wedge 95 holds the permanent magnets 85 in place against the rotor housing 92.
One drawback of the circumferential as compared to the radial-oriented magnet arrangement is that the leakage flux is comparatively larger, as shown in FIG. 3. This leakage flux is that magnetic flux which does not cross the air gap and link the stator winding, thus providing no useful magnetic field. Both the upper rotor leakage flux path and the lower rotor leakage flux path are depicted in FIG. 3 as flux lines. This leakage flux is typically reduced by making the rotor core or shaft non-magnetic. A reduction in leakage flux would result in a corresponding reduction in the amount of magnet volume required to produce a given air gap flux density.
As briefly described above, the stator configuration for both types of rotor topologies may be the same. The stator generally consists of a slotted core 70 with a polyphase winding 80 similar to that in an induction motor or wound field synchronous machine.
The circumferential magnet arrangement (or “spoke topology”) is well suited for propulsion applications. This is due, in part, to its superior resistance to demagnetization of the permanent magnets 85 resulting from either armature reaction or excessive temperature. Only a small component of the demagnetizing magnetic field appears across the permanent magnets 85 as a result of the armature ampere turns because the iron rotor pole head 90 provides an alternative low reluctance parallel path across the air gap (referred to as the q-axis path) so that the magnets 85 are partially shielded from the armature demagnetizing currents. Consequently, the circumferential magnet configuration is considered more stable and less sensitive to demagnetizing magnetic field effects than its radial counterpart.
However, a disadvantage of the conventional spoke circumferential topology as compared to the surface mounted radial magnet design is its poorer utilization of the magnetic material or magnets. The high proportion of leakage flux limits the torque production; it takes a greater volume of magnetic material to produce a given amount of torque. This is due in part to the greater amount of leakage flux in this design as shown in FIG. 3. As seen in the figure, a large percentage of the leakage flux occurs at the inner radius of the permanent magnets 85, which heretofore are of a rectangular shape. The circumferential magnet or spoke topology using rectangular shaped magnets 85 thus does not compare well against surface mounted permanent magnets 15 in FIG. 1(A) because the high proportion of leakage flux limits torque production.
Therefore, the surface mounted (radial) magnet topology can give the required electromagnetic torque with less leakage and thus, a smaller magnet volume than the spoke (circumferential) design, but the radial design is less desirable in several applications when torque harmonics, acoustic performance, permanent magnet surface eddy current losses, and resistance to demagnetization are considered. Thus, there is a desire or need to increase the torque density of spoke-type synchronous machine topologies. As such, the present invention, in at least one preferred embodiment, addresses one or more of the above-described and other limitations to the prior art using an approach to improve the torque per volume ratio of the spoke topology motor as compared to the more conventional surface mounted designs.